This invention relates to platform positioning, i.e., tracking and locating points relative to an earth coordinate system, and, more particularly, to a method and apparatus for deriving pseudo range from earth-orbiting, signal-transmitting satellites.
The U.S. Navy TRANSIT Navigational Satellite System comprises a number of satellites in near-earth polar orbit. The satellites transmit to user stations 150 and 400 MHz carriers on which satellite ephemeris information is modulated. The user measures the frequency of the received signal and calculates his range with respect to transmitting satellites based on the Doppler frequency shift observed by the user and the satellite ephemeris information. Specifically, if the positions of the observed satellite and its transmission frequency are known, the Doppler frequency shift of the signals received from the satellite at a point as it passes over the receiver in 15 minutes or less permits the determination of the location of that point in an earth-referenced coordinate system. A further description of TRANSIT is given in an article by H. D. Black entitled "Satellites for Earth Surveying and Ocean Navigating," Johns Hopkins APL Technical Digest, January-March, 1981, Volume 2, No. 1, pages 3-13.
The NAVSTAR Global Positioning System, which comprises a plurality of coded signal transmitting satellites in far earth orbit, will provide, to authorized users in possession of the code, greater accuracy and more flexibility than TRANSIT. Although at present there are only six NAVSTAR satellites in orbit, it is planned that eventually there will be 18, so distributed that four satellites are visible from any point on earth. Each NAVSTAR satellite transmits a so-called L.sub.1 band radio signal centered about a frequency of 1575.42 MHz and an L.sub.2 band radio signal centered about a frequency of 1227.6 MHz. The L.sub.1 band has a suppressed carrier L.sub.1 upon which a protected information channel called P-code and a coarse acquisition channel called C/A code are modulated in double side band form. The L.sub.2 band has a suppressed carrier L.sub.2 upon which the P-code is modulated in double side band form. The P-code and C/A code channels both carry binary information signals at a given chip rate, i.e., 10.230 MHz in the case of the P-code, and 1.0230 MHz in the case of the C/A code. The P-code sequence is only known to authorized users of the NAVSTAR system. With knowledge of the P-code sequence, an authorized user of NAVSTAR can derive his pseudo range relative to the satellites in his field of view by measuring the time it takes for the P-code sequence to traverse the distance between the satellites and the user, knowing that the signal travels at the speed of light. The true range of the user from a satellite equals the pseudo range plus the difference in the offset of the satellite clock and the user clock relative to a time reference such as the universal time coordinated scale. Having thus measured the pseudo range, a user can determine the satellite and user clock offsets and perform a number of tracking and location determinations, termed "platform positioning," by means of well-known techniques. To those not having access to the P-code sequence, the NAVSTAR satellite system cannot currently be used to determine high-precision pseudo range.